The invention relates to a method for controlling a driver assistance system. The invention also relates to a corresponding driver assistance system and a computer program product.
Recent advances in assisting a driver operating a vehicle have successfully emerged into the market as they compensate shortcomings of human drivers, such as inevitable reaction times for emergency brakes and/or deficiencies for vehicle stabilization. The anti-lock braking system (ABS) and the electronic stability program (ESP) used in modern vehicles are two technologies for assisting the driver during a critical operational situation.
In the event that the vehicle is braking, under some conditions the forces that are generated can result in unwanted reaction forces being applied to the vehicle and the steering system. At best these may just be felt through the wheel, as a mild disturbance, which gives an uncomfortable and possibly alarming feel to the driver. At worst, the forces can unbalance the vehicle causing it to spin out of control and possibly disturb the driver with a heavy steering wheel torque disturbance.
One situation in which this can occur is known as a split-friction braking event, also known as split-mu, where the coefficient of friction is different at the two vehicle sides. The braking force in such an event may be substantially different between left and right vehicle sides. This asymmetry and imbalance of the effective brake forces on the right and the left vehicle sides will generate a yaw torque that turns the vehicle about its vertical axis depending on these asymmetric forces. To counteract this condition and preserve the directional and driving stability, i.e. to keep the vehicle on course, the driver would have to manipulate the steering wheel in this situation with an extremely quick reaction for correcting purposes.
The yaw torque may be limited by reducing the brake force, which will result in an unwanted increase in stopping distance for the vehicle. There is thus a necessity to balance the applied brake forces, at the different wheels, in such a manner that the driving stability is maintained while at the same time reducing the stopping distance. However, priority is generally given to the preservation of the directional and driving stability as well as the steerability of the vehicle over reaching shortest possible stopping distance.
There is thus a desire to provide means for further reducing the stopping distance while keeping driving stability. EP1209053 provides some relief to this problem by suggesting an electric steering control strategy to assist the driver in controlling the vehicle during a split-mu braking operation. A correction angle of deflection of the steered wheels is calculated to compensate for the yaw torque for maintaining the vehicle in its path. The correction angle is then used for controlling an engine of the electric power steering and hence the steering wheel. Accordingly, driver feels, and is encouraged, to turn the steering wheel to compensate for the unwanted yaw torque.
Even though EP1209053 provides for reducing the stopping distance while keeping driving stability, it would be desirable to introduce an improved methodology for assisting the driver of a vehicle during a split-mu braking event, where the methodology takes the driver situation and operation further into account.
According to an aspect of the invention, a method is provided for controlling a driver assistance system that includes a brake support function of a vehicle, the method comprising the steps of receiving an indication of a difference in road friction between a left and a right wheel of a plurality of wheels of the vehicle, receiving an indication of a level of driver reaction to a braking action, the braking action performed by applying a predetermined brake pressure to the plurality of wheels of the vehicle, and determining an adjusted brake pressure based on the difference in road friction and the driver reaction level.
In accordance with an aspect of the invention, a correlation is made between a difference in road friction and how a driver is reacting to a braking action, where the result of the correlation will result in a request for an adjustment of the brake pressure. As an example, this means that in case the braking action results in a yaw torque that turns the vehicle about its vertical axis because of a current difference in friction, the braking action can be adjusted to take into account the action taken by the driver. The inventor has shown that the stopping distance is very much affected by the capability of the driver to handle the lateral disturbance. In general the capability of the driver to handle disturbances should be estimated in real-time, considering driver mental state.
Thus, in case the driver is counteracting the yaw torque, the brake pressure can be increased, typically shortening the stopping distance. However, in case the driver is not or only slightly counteracting the yaw torque, the brake pressure may be adapted to focus on keeping the vehicle as straight as possible, however with a possible (in comparison) longer stopping distance. The invention may typically be implemented in relation to an automated or semi-automated braking function for the vehicle. Alternatively, or also, the invention may be implemented as a response to a manual braking imitated by the driver. The adjusted brake pressure is preferably further based on an allowed yaw disturbance for the vehicle, typically based on current thereto related legislations.
In an embodiment of the invention, the predetermined brake pressure is determined based on an indication of a level of driver alertness. Accordingly, it may be possible to allow the initial brake pressure (when automatically performing a braking action) to depend on an indication of how alert the driver is. The expectation is that the driver would be more likely to be prepared to counteract the yaw torque in case he is determined to be alert, and less likely to be able to counteract the yaw torque in case of being less alert. For example, a “sleepy” driver is likely not as responsive as if the driver just had a pause from driving the vehicle. It could be possible, and within the scope of the invention, to allow an alertness level to directly correspond to the brake pressure, however it could also be possible to allow for the brake pressure to be somewhat functionally amplified for higher alertness levels. In a possible embodiment of the invention a driver drowsiness detection system is comprised with the vehicle and continuously provides information as to a current level of driver alertness for the driver.
Preferably, the driver reaction level includes at least one of an indication of a driver operation of a driver steering device or a brake pedal of the vehicle. Thus, and as indicated above, in case the driver is actively working for counteracting the yaw torque, by e.g. rotation of a steering wheel, this may be seen as an indication of an in comparison high level of driver reaction. Similarly, in case the driver is actively using the brake pedal for retarding the vehicle, this is in accordance with the invention ranking as an active driver, thus scoring an in comparison high level of driver reaction. In comparison, in case the driver is refraining from using the steering wheel and/or the brake pedal, this would be scoring as an in comparison low level of driver reaction. The expression driver steering device should be interpreted broadly, including e.g. the above mentioned steering wheel and/or any other type of levers and/or similar for adjusting a direction of the vehicle. The expression brake pedal should similarly be interpreted broadly, and may not necessarily relate to a device for receiving a pressure by a foot of the driver. Rather, any type of levers or similar for achieving a manual brake force is to be understood as included.
In a preferred embodiment of the invention, the brake pressure is continuously adjusted during the braking action. Accordingly, the adjustment will continuously be dependent on any changes as the difference in road friction and/or to the driver reaction. Thus, in implementing the inventive concept it may be possible to compensate for a somewhat slow reacting driver, i.e. even in case the driver didn't react immediately to the generated yaw torque, once the driver is in fact reacting, the brake pressure may be adjusted accordingly. Similarly, in case the difference in road friction is changing, over the road travelled, the brake pressure may be adjusted. It is preferred to determine and adjust the brake pressure independently for each of the plurality of wheels of the vehicle.
As an extension of the inventive concept, it may be possible to further take into account an indication of a road condition ahead of the vehicle and within a predefined area, the road condition requiring the braking action. Thus, in case e.g. a safety functionality implemented with the vehicle identities an obstacle ahead of the vehicle, this information may be taken into account by the inventive concept. The predetermined area may for example be within 1000 meters in front of the vehicle. Other distances are of course possible and within the scope of the invention.
Typically, an activation time for performing the braking action is based on the road condition and the driver alertness level. For example a distance to the obstacle may dictate when to initiate the braking action. Other possible road conditions include upcoming known problematic road sections, curves, intersections, traffic congestions, etc. The indication of a road condition ahead of the vehicle may also possibly be obtained from (previously) received/collected e-horizon information. The e-horizon information could include map data, information about traffic works, hills, etc.
Furthermore, it may in accordance to the invention be possible to determine a desired driver steering device position for the vehicle when performing the braking action, and automatically applying an assisting driver steering device force, where the assisting driver steering device force is based on the desired driver steering device position and the driver reaction level. Accordingly, in case the driver is counteracting the yaw torque by e.g. turning the steering wheel, there could be possible to further assist the driver performing the counteracting action. In an embodiment, this is achieved by means of controlling an engine of an electric power steering to apply an assisted steering wheel torque corresponding to a desired steering wheel angle. In some embodiments also e.g. a small rotation of the steering wheel in the correct direction may be used as an indication of a driver reaction, and the assisting driver steering device force will further help the driver to achieve the desired result of counteracting the yaw torque of the vehicle.
According to another aspect of the present invention there is provided a driver assistance system that includes a brake support function of a vehicle, the vehicle assistance system comprising a control unit, wherein the control unit is configured to receive an indication of a difference in road friction between a left and a right wheel of a plurality of wheels of the vehicle, receive an indication of a level of driver reaction to a braking action, the braking action performed by applying a predetermined brake pressure to the plurality of wheels of the vehicle, and determine an adjusted brake pressure based on the difference in road friction and the driver reaction level. This aspect of the invention provides similar advantages as discussed above in relation to the previous aspect of the invention.
According to a still further aspect of the present invention there is provided a computer program product comprising a computer readable medium having stored thereon computer program means for operating a driver assistance system that includes a brake support function of a vehicle, the computer program product comprising code for receiving an indication of a difference in road friction between a left and a right wheel of a plurality of wheels of the vehicle, code for receiving an indication of a level of driver reaction to a braking action, the braking action performed by applying a predetermined brake pressure to the plurality of wheels of the vehicle, and code for determining an adjusted brake pressure based on the difference in road friction and the driver reaction level. Also this aspect of the invention provides similar advantages as discussed above in relation to the previous aspects of the invention.
The computer readable medium may be any type of memory device, including one of a removable nonvolatile random access memory, a hard disk drive, a floppy disk, a CD-ROM, a DVD-ROM, a USB memory, an SD memory card, or a similar computer readable medium known in the art.
Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.